On October 13, 2011, Valve switched the Noh Mercy from a Hat to a miscelleneous item. When they did that, they (presumably unintentionally) accidentally removed it from the unbox pool, meaning that from that day onward you can no longer uncrate them. On the day of that update, the amount of unusual Noh mercies was effectively frozen with no more to ever come since. This is a bold claim; and can be backed up by checking the original ID on any Noh Mercy unusual. The original ID will always date before Oct 13, 2011, (however some item tracking sites, being imperfect and unable to see into private backpacks may show later dates.)
This claim can be further proven by examining the unusual effects available on the Noh Mercy. Of the 48 unusual effects out there, only 22 can be seen on the Noh Mercy. None of the halloween effects (Flaming Lantern, Harvest moon, etc) exist, since the 2011 Halloween event started just a few weeks after October 13th. None of the 3rd Generation (Aces high, Disco beat down, etc) or Robotic (Antifreeze, Timewarp, etc) are available either, as those were released in 2013 - years after it's been discontinued.
There's a finite amount to exist; no more are coming. Since these unusuals are all very old (remember, all are older than Oct 2011), a good portion of Noh Mercy unusuals reside in trade banned, account restricted, or simply inactive/forgotten accounts, never to be recovered. Others are locked away in Spy collector backpacks. The very few up for trade always generate a lot of interest, and those lucky enough to own one are never quick to let go of it. 2013 in particular has shown a huge spike in Noh Mercy unusual value, as more and more people become educated that they're no longer being unboxed.